Spectroscopy of three-particle entanglement in a macroscopic superconducting circuit.

We study the quantum mechanical behavior of a macroscopic, three-body, superconducting circuit. Microwave spectroscopy on our system, a resonator coupling two large Josephson junctions, produced complex energy spectra well explained by quantum theory over a large frequency range. By tuning each junction separately into resonance with the resonator, we first observe strong coupling between each junction and the resonator. Bringing both junctions together into resonance with the resonator, we find spectroscopic evidence for entanglement between all 3 degrees of freedom and suggest a new method for controllable coupling of distant qubits, a key step toward quantum computation.